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White matter
Hemispheric white matter tracts divided by course, connections into association, commissural, projection fibers Association fibers (may be short or long) * Short (arcuate or "U" fibers) link adjacent gyri, course parallel to long axis of sulci * Long fibers form fasciculi connecting widely spaced gyri ** Cingulum: Long, curved fasciculus deep to cingulate gyrus; interconnects parts of frontal/parietal/temporal lobes ** Uncinate fasciculus: Connects motor speech area & orbital gyri of frontal lobe with temporal lobe cortex ** Superior longitudinal (arcuate) fasciculus: Connects frontal to parietal, temporal and occipital cortex ** Inferior longitudinal fasciculus: Connects temporal and occipital cortex, contributes to sagittal stratum ** Superior occipitofrontal fasciculus: Connects occipital & frontal lobes, lies beneath corpus callosum (CC) ** Inferior occipitofrontal fasciculus: Connects occipital & frontal lobes, inferiorly; posteriorly forms sagittal stratum which connects occipital lobe to rest of brain Commissural fibers * Corpus callosum ** Largest commissure; links hemispheres ** Four parts: Rostrum, genu, body, splenium ** Rostral fibers extend laterally connecting orbital surfaces of frontal lobes ** Genu fibers curve forward as forceps minor, connect lateral/medial frontal lobes ** Body fibers (tapetum) pass laterally, around posterior and inferior horns of the lateral ventricle, intersect with projection fibers of corona radiata to connect wide areas of hemispheres. ** Most fibers from splenium curve into occipital lobes as forceps major * Anterior commissure ** Transversely oriented bundle of compact myelinated fibers ** Relations: anterior to fornix, embedded in anterior wall of third ventricle ** Splits into two bundles laterally ** Anterior bundle to anterior perforated substance, olfactory tract. Larger posterior fans out into temporal lobe * Posterior commissure: Small; courses transversely in posterior pineal lamina to connect midbrain, thalamus/hypothalamus. Connects the superior colliculi for light reflex. * Hipopocampal: '''the commissure of the fornix Projection fibers * '''Corona radiata: Fibers from internal capsule fan out to form corona radiata, represent all cortical areas * Internal capsule: Major conduit of fibers to/from cerebral cortex ** Anterior limb: Frontopontine fibers, thalamocortical projections ** Genu: Corticobulbar fibers ** Posterior limb: Corticospinal tracts, upper limb-anterior, trunk & lower limbs-posterior * Corticospinal tract: efferent fibres arise from upper motor neurons in the precentral gyrus and premotor cortex. ** Converge into corona radiata, continue through posterior limb of internal capsule to cerebral peduncle ** The fibres separate into bundles through the pons and converges again in the medulla. ** At the tail end of the medulla, 90% decussate and form the lateral corticospinal tract. ** 10% non-decussated fibres form the anterior corticospinal tract. * Corticobulbar tract: efferent projection fibers from the upper motor neuron in the precentral gyrus (face region) ** Converge into corona radiata to genu of internal capsule to cerebral peduncle, terminate in motor cranial nerve nuclei * Corticopontine tract: Motor information to pons * Corticothalamic tract: Connects entire cerebral cortex with isotopic location in thalamus * Spinothalamic tract ** Pain and temperature pathway ** Fibres enter via the dorsal root ganglion and decussate immediately before ascending in the spinothalamic tract in the anterolateral quadrant of the cord (the sacral fibres are most lateral and the cervical fibres are most medial) ** It ascends in the lateral part of the medulla and pons, mingling with the medial lemniscus in the rostral midbrain ** Both the spinothalamic tract and medial lemniscus terminate in the VPL (ventral posterolateral) nucleus of the thalamus, giving off thalamocortical fibres that pass laterally to enter the anterior limb of the internal capsule, terminating in the post-central gyrus * Dorsal column/medial lemniscus system ** Light touch, vibration and proprioception pathway ** Fibres enter the spinal cord via the dorsal root ganglia and ascend in the dorsal columns (those entering below the midthoracic level form the gracile tract medial and those entering above the midthoracic level form the cuneate tract lateral) ** The tracts ascend to the brain without crossing and occupy most of the dorsal part of the dorsal column ** The axons of the gracile and cuneate tracts terminate in the gracile and cuneate nuclei (tubercles) within the caudal medulla where they cross the midline as the sensory decussation and ascend in a bundle called the medial lemniscus ** The medial lemniscus borders the midline of the medulla and contains axons from the gracile nucleus in its anterior half and cuneate nucleus in its posterior half which then twists in the pons with the lower limb (gracile) nucleus represented laterally and the upper limb (cuneate) nucleus represented medially ** The medial lemniscus passes to the VPL, then via the posterior limb of the internal capsule to terminate in the post-central gyrus and posterior part of the paracentral lobule * Optic tract * Spinocerebellar tract ** A set of axonal fibres originating in the spinal cord and ascending to the ipsilateral cerebellum where they terminate ** Contains proprioceptive information * Rubrospinal tract ** Fibres from the red nucleus (in the tegmentum of the midbrain at the level of the superior colliculus and pretectum) cross the midline and descend through the brainstem facilitating flexor movements in the contralateral upper limb * Trigeminothalamic tract ** Pathway fro pain, temperature and light touch from the face, head and neck ** Receives input from the trigeminal nerve, facial nerve, glossopharyngeal nerve and vagus nerve and terminates in the VPL of the thalamus on the contralateral side, ascending to the sensory cortex * Reticulospinal tract ** Extrapyramidal motor tract which travels from the reticular formation and functions to integrate information from the motor systems to co-ordinate automatic movements of locomotion and posture ** Facilitates and inhibits voluntary movement and influences muscle tone